Stainless steel has been used in various applications in recent years, exploiting its excellent corrosion resistance. Local corrosions such as pitting corrosion, crevice corrosion, and stress corrosion cracking are particularly important with regard to the corrosion resistance of components such as stainless steel devices or pipes, and there is a problem that these give rise to penetration holes through which internal fluids can leak.
In marine environments, airborne salt which includes a large amount of seawater components is the corrosive element. In cold regions, chlorides contained in antifreezing agents which are spread in winter are the corrosive element. Sodium chloride and magnesium chloride are present as chlorides contained in seawater. These chlorides become adhered as an airborne salt component. When they then become wet, they readily form concentrated chloride solutions. Meanwhile, antifreezing agents are formed of calcium chloride and sodium chloride, and since they are typically applied in a solid state, they readily form a concentrated chloride solution. Among the chlorides varieties, sodium chloride dries at a relative humidity of 75% or less, while magnesium chloride and calcium chloride will not dry until the relative humidity reaches 40% or less. As a result, magnesium chloride and calcium chloride form concentrated chloride solutions over a wider humidity range. This also expresses the extent of deliquescence, showing that magnesium chloride and calcium chloride absorb moisture at a lower humidity to form a concentrated chloride solution, compared with sodium chloride. Since the relative humidity is typically in the range of 40 to 75% in ambient air, it is extremely important to have a superior corrosion resistance in the presence of concentrated magnesium chloride or concentrated calcium chloride.
Patent Document 1 discloses a ferritic stainless steel with improved resistance to crevice corrosion. The invention disclosed in this specification is characterized in obtaining superior resistance to crevice corrosion by adding a mixture of 16% or more of Cr and about 1% of Ni, without requiring a large addition of Cr or Mo. In this Patent Document 1, evaluation was carried out using a repeated drying and wetting test in a sodium chloride environment. By employing a repeated drying and wetting test, the corrosion characteristics of the disclosed ferritic stainless steel in a concentrated sodium chloride solution can be ascertained; however, no consideration is given to the corrosion properties in a solution of concentrated magnesium chloride or concentrated calcium chloride.
Patent Document 2 discloses a ferritic stainless steel which can be used in marine environments due to the addition of a large amount of Cr and Mo, and a suitable amount of Co. However, Co and Mo are expensive and manufacturability is impaired with the addition of large amounts of Cr, Mo, and Co. Patent Document 3 discloses a ferritic stainless steel in which corrosion resistance is improved by the addition of P, and therefore, large amounts of Cr and Mo are not required. Furthermore, by optimizing amounts of C, Mn, Mo, Ni, Ti, Nb, Cu and N, manufacturability can be assured. However, since P causes a deterioration in welding properties, this is a hindrance when manufacturing welded structures. Further, the most severe test of corrosion resistance that is disclosed in Patent Document 3 is the CASS test (sodium chloride solution spray test), and no consideration is given to concentrated magnesium chloride or concentrated calcium chloride environments. Patent Document 4 discloses a ferritic stainless steel in which corrosion resistance is increased by the addition of P, and the improvement of cleanness and the control of configuration of inclusions are aimed to be attained by adding suitable amounts of Ca and Al. This Patent Document 4 also discloses selective addition of Mo, Cu, Ni, Co and the like. Here, the most severe corrosion test is a crevice corrosion generating test conducted in 10% ferric chloride-3% sodium chloride solution, and no consideration is given to concentrated magnesium chloride or concentrated calcium chloride environments.
Austenitic stainless steel typified by SUS304 and SUS316L has excellent resistance to penetration hole formation caused by pitting corrosion or crevice corrosion, but there is concern with respect to its resistance to stress corrosion cracking. Accordingly, so-called “super” austenitic stainless steel which includes high-Cr, high-Ni, and high-Mo to suppress the occurrences of the pitting corrosion and the crevice corrosion that are the causes of the stress corrosion cracking may be considered to be employed, or SUS315J1, 315J2 type steels in which stress corrosion cracking is improved by combined addition of Si and Cu may be considered to be employed. However, both of these approaches are expensive.
Ferritic stainless steel has come to be used in various applications in recent years due to its corrosion resistance, formability, and cost performance. Local corrosions such as pitting corrosion, crevice corrosion, and stress corrosion cracking are particularly important with respect to durability of stainless steel equipments and pipings. For ferritic stainless steels, pitting corrosion and crevice corrosion are particularly important. In the case of components where crevice portions are present in the design at welded sites, flange attachment sites, and the like, crevice corrosion is particularly important, and there is a problem that this crevice corrosion gives rise to penetration holes through which internal fluids may leak. For example, in the case of automobiles, there is a move to extend the guarantee period from 10 to 15 years for essential parts such as fuel tanks, fuel supply lines, and the like, and therefore, there is a need to ensure reliability over a long period of time.
Further, local corrosions as described above are also important for the durability of stainless steel equipments and piping components which are employed in chloride environments.
In order to prevent penetration holes due to crevice corrosion, and damage due to stress corrosion cracking arising from crevice corrosion, Patent Documents 5 and 6 disclose counter measures using coating and sacrificial corrosion protection.
In the case of coatings, there is a large burden on the environmental measures since solvents and the like are used in the pre-treatment process. Further, in the case of sacrificial corrosion protection, there is a problem where maintenance costs are expensive. Therefore, it is desirable to ensure resistance to crevice corrosion in an untreated state without relying on coating or sacrificial corrosion protection. Employment of a ferritic stainless steel in which corrosion resistance is improved by adding large amounts of Cr and Mo may be considered as one approach. However, steels which include high-Cr and high-Mo have a problem that formability is inferior and, moreover, are expensive. Therefore, a material which has both of corrosion resistance and formability without the addition of a large amount of an expensive element such as Mo has been desired.
Patent Document 7 discloses a ferritic stainless steel in which corrosion resistance is increased by the addition of P, and the improvement of cleanness and the control of configuration of inclusions are aimed to be attained by adding suitable amounts of Ca and Al. This Patent Document 7 further discloses the selective addition of Mo, Cu, Ni, Co and the like. However, the P causes a deterioration in welding properties, and is thus a hindrance when manufacturing welded structures. Further, costs rise due to the deterioration in manufacturability. Further, while suitable amounts of Ca and Al may be added to augment the decline in formability due to P, the suitable range is narrow, and production costs increase. Therefore, the ferritic stainless steel becomes expensive, and the merit of employing ferritic stainless steel is diminished due to its high cost as a material.
The above described Patent Document 1 discloses a ferritic stainless steel in which resistance to crevice corrosion is improved by the addition of Ni, and discloses the selective addition of Mo and Cu for the purpose of further improving resistance to crevice corrosion. Because Ni decreases formability, there is a problem that it becomes difficult to form components where a high degree of formability is required, such as exhaust components or fuel system components of automobiles.
With regard to ferritic stainless steels containing Sn and Sb, a ferritic stainless steel plate having excellent high temperature strength is disclosed in Patent Document 8, while a ferritic stainless steel having excellent surface properties and corrosion resistance, and a method for manufacturing the ferritic stainless steel are disclosed in Patent Documents 9 and 10. In Patent Document 8, improvement in high temperature strength, and, in particular, a prevention of a deterioration in high temperature strength after long time aging is raised as the effect of Sn. Similar attributes are ascribed to Sb. The effect in the present invention is an effect to the resistance to crevice corrosion, and differs from the effects of Sn and Sb in Patent Document 8. In contrast, Patent Documents 9 and 10 are characterized in employing Mg and Ca as bases, adding Ti, C, N, P, S and O, and then controlling the contained amounts of these elements to improve ridging characteristics and corrosion resistance. Sn is disclosed as a selectively added element. Improvement of corrosion resistance is raised as the effect of Sn, and the corrosion resistance is evaluated using pitting potentials in the examples. The pitting potential electrochemically evaluates resistance with respect to the generation of pitting corrosion. In contrast, crevice corrosion is the subject of study in the present invention. As will be explained below, one aspect of the present invention uncovers, as the efficacy of Sn, an effect of limiting progression after the generation of crevice corrosion, and is different from the effect of improving resistance to the generation of pitting corrosion which is disclosed in
Patent Documents 9 and 10.
Patent Document 1:Japanese Patent Application, First Publication No. 2005-89828Patent Document 2:Japanese Patent Application, First Publication No. S55-138058Patent Document 3:Japanese Patent Application, First Publication No. H6-172935Patent Document 4:Japanese Patent Application, First Publication No. H7-34205Patent Document 5:Japanese Patent Application, First Publication No. 2003-277992Patent Document 6:Japanese Patent No. 3545759Patent Document 7:Japanese Patent No. 2880906Patent Document 8:Japanese Patent Application, First Publication No. 2000-169943Patent Document 9:Japanese Patent Application, First Publication No. 2001-288543Patent Document 10:Japanese Patent Application, First Publication No. 2001-288544